Hello! I bought a Plextor 40/12/40 about five months ago.I run it with Windows XP and I'm currently using "Nero" burning rom software.Everything has been working perfect until just recently.Suddenly I'm not able to record CDR's very good.
If I'm lucky I can get it to work burning 4x the speed but any thing above this speed is not working o.k.
I've tried everything:
I reinstalled Windows XP from scratch(reformatted) and before anything else I installed "Nero" and tried, didn't work.
I upgraded to the latest firmware, didn't work.
I tried different versions of burning software, didn't work.
I tried different IDE channels and setup's, didn't work.
The problem is that when the burning cycle reaches 90-99%
it says "writing error" , "Invalid write state" and "Could not perform end track".
I also tried different brands of CDR's, didn't help.
If I try the simulation it's always successful

Nero does a media assessment and will automatically reduce the burn speed it the media isn't capable of the speed selected. As you state that you have to go down to 4x & have tried different media it didn't sound like a media fault.
As you've elimiated software and media that only leaves hardware. It's still under warranty so get Plextor to check it.